Two general types of electric propulsion thrusters used in space include ion thrusters and Hall-effect thrusters.
Both ion thrusters and Hall-effect thrusters are ‘electrostatic’ electric propulsion devices used for spacecraft propulsion that create thrust by accelerating ions. The thrust created is very small compared to conventional chemical rockets, but a very high specific impulse, or high exhaust velocity, which reduces the propellant requirements for missions is obtained. This high ‘propellant efficiency’ is achieved through the very frugal propellant consumption of the electric propulsion system. They do however require large amounts of power; typically 1 kWe per 0.030-0.040 Newtons thrust for ion thrusters, and 1 kWe per 0.050-0.080 Newtons thrust for Hall-effect thrusters.
Ion thrusters and Hall-effect thrusters both generate a beam of ions (electrically charged atoms or molecules) to create thrust in accordance with Newton's third law. The method of accelerating the ions varies, but all designs take advantage of the charge/mass ratio of the ions. This ratio means that relatively small potential differences can create very high exhaust velocities. This reduces the amount of reaction mass or fuel required, but increases the amount of specific power required compared to chemical rockets. Electric propulsion thrusters are therefore able to achieve extremely high specific impulses.
The drawback of the low thrust is low spacecraft acceleration because the mass of current electric power units is directly correlated with the amount of power required. This low thrust makes electric propulsion unsuited for launching spacecraft into orbit, but they are ideal for in-space propulsion applications.
Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from an electron source as a hot cathode filament, or more typically a hollow cathode assembly (HCA), which are then accelerated in the electrical field of the cathode fall to the anode (Kaufman type ion thruster).
The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system via the plasma sheath the ions are accelerated due to the potential difference between the first and second grid (named screen and accelerator grid) to the final ion energy of typically 1-2 keV, thereby generating the thrust. Typical ion velocities are in the range of 20,000-50,000 m/s, and higher for some energetic mission applications.
In spacecraft propulsion, a Hall-effect thruster also accelerates ions by an electric field. Hall-effect thrusters trap electrons in a radial magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the ions in the plume.
The essential working principle of the Hall-effect thruster is that it uses an electrostatic potential to accelerate ions up to high speeds but does so without the application of a gridded extraction system used in ion thrusters. In a Hall-effect thruster the attractive negative charge is provided by an electron plasma at the open end of the thruster instead of a grid. A radial magnetic field of a few tens of milli-Tesla is used to confine the electrons, where the combination of the magnetic field and an attraction to the anode upstream surface force a fast circulating electron current around the axis of the thruster and only a slow axial drift towards the anode occurs.
A propellant, such as xenon gas is fed through the anode, which has numerous small holes in it to act as a gas distributor. As the neutral xenon atoms diffuse into the channel of the thruster, they are ionized by collisions with high energy circulating electrons.
The xenon ions are then accelerated by the electric field between the anode and the cathode. The ions quickly reach speeds of around 15,000 m/s for a specific impulse of 1,500 seconds (15 kN·s/kg). Upon exiting however, the ions pull an equal number of electrons with them, creating a plume with no net charge. The axial magnetic field is designed to be strong enough to substantially deflect the low-mass electrons, but not the high-mass ions which have a much larger gyroradius and are hardly impeded. About 30% of the discharge current is an electron current which does not produce thrust, which limits the energetic efficiency of the thruster; the other 70% of the current is in the ions. The ionization efficiency of the thruster is thus around 90%.
The magnetic field thus ensures that the discharge power predominately goes into accelerating the xenon propellant and not the electrons, and the thruster turns out to be reasonably efficient.
Because of the counter-flowing electron and ion currents in the Hall-effect thruster channel, a greater ion flux can be achieved as compared to that of the ion thruster—thereby yielding higher thrust-to-power than ion thrusters. Ion thrusters however are capable of achieving higher exhaust velocities with higher overall thrust efficiencies.